Core lock (or rotor lock) is a condition that can occur in non-operating gas turbine propulsion engines on aircraft that are in flight. In particular, in the unlikely event that a gas turbine propulsion engine experiences a flameout during flight, and if sufficient airflow is not maintained through the engine to keep the engine core rotating, differential cooling of the engine core may occur. Typical gas turbine propulsion engines are constructed of many different materials, both metal and non-metal, that cool at slightly different rates. Moreover, typical gas turbine propulsion engines are precision built. Thus, this differential cooling rate can cause a rotor lock condition. In some circumstances, once a rotor lock condition occurs, it may not be possible to restart the engine.
As most aircraft pilots can appreciate, a rotor lock condition can inhibit or prevent either a windmill restart or an APU assisted restart of the engine. Indeed, in at least one particular aircraft incident a rotor lock condition was cited as a contributing cause. During that particular flight, both propulsion engines experienced a relatively high altitude flameout. As the aircraft drifted down to the engine re-light envelope, the flight crew did not maintain sufficient airspeed to keep the engine core rotating. As a result, both propulsion engines experienced rotor lock conditions and could not be restarted.
In response to the above-described incident, regulatory agencies began looking for solutions to prevent propulsion engine rotor lock in aircraft. Thus far, investigations have concentrated mainly on engine solutions and flight manual solutions. Unfortunately, engine solutions are likely to be rather difficult to implement. Flight manual solutions, while being relatively less difficult to implement, may not be sufficiently pragmatic, as this solution would rely on a flight crew referencing and reading a flight manual during a highly stressful situation. One further exacerbating factor associated with propulsion engine rotor lock prevention, is the somewhat counter-intuitive action that will prevent a rotor lock condition. That is, for most flight crews the intuitive action following a flameout of both engines is to fly the aircraft relatively slowly for maximum glide range. However, the needed action is to maintain aircraft speed above a sufficient speed to keep the propulsion engines, and most importantly the engine cores, rotating.
Hence, there is a need for a system and method of preventing, or at least inhibiting the likelihood of occurrence of, aircraft gas turbine propulsion engine rotor lock conditions that does not rely on relatively complex engine solutions and/or aircraft flight manual solutions and/or assists aircraft flight crews in taking potentially counter-intuitive actions. The present invention addresses one or more of these needs.